This invention relates to fluorescent display tubes of the type commonly employed to display data in numeric and/or letter form.
A typical form of fluorescent display tube includes an anode substrate coated with a fluorescing substance (FIG. 1), a grid spacer (FIG. 2) placed over the substrate and the mesh of a grid electrode (FIG. 3) laid over the grid spacer. Other parts such as a cathode and a support structure as assembled on the stack to complete an electrode assembly, and a glass covering is sealingly secured to the electrode assembly. Such configuration is fully described by Takao Kishino in U.S. Pat. No. 4,047,073 (Patented on Sept. 6, 1977). As is well known, the display tube is evacuated at the last stage of manufacture through a gas exhausting tube attached thereto. The anode arrangement of the display tube includes a number of segmented luminescent anodes or groups of anode segments and, in operation, only those anode segments which are held at positive potential are excited to emit light by bombardment of the electrons emitted from the cathode and accelerated by the control-grid potential, as the anode segments in each group are selectively excited to display a numeral or letter desired.
In other words, in operation of the fluorescent display tube, the anode segments in each group never emit light all at the same time and the combination of anode segments selected or excited to emit light by electron bombardment usually varies from time to time to display different numerals or letters in turns. In some case, any of the anode segments in some group or other is not excited so that there is no display in the digit place. Specifically, with the conventional mesh pattern of grid electrode (FIG. 3), among the electrons emitted from the cathode and proceeding through the mesh under the acceleration effect of the grid potential, those electrons approaching the anode segments held at positive potential excite such segments effectively and thereby cause them to emit light, but the remaining electrons passing through the mesh and approaching the other segments, not held at positive potential, and the exposed insulating surface (FIG. 1) of the anode substrate are attracted back to the grid electrode. Such behavior of electrons, occurring in repetition, gives rise to a sort of oscillation, which is known as Barkhausen oscillation. Such oscillation is particularly observed in the VHF range, though limited in intensity. In cases where such display tubes are employed with televisions or other electronic appliances utilizing such frequency range, weak electric waves resulting from the Barkhausen oscillation give rise to practical problems, impeding the performance of the associated appliance to a marked extent.